Four kaka at the Pukaha Mount Bruce Wildlife Centre have had to be euthanised after developing hereditary diabetes.
The juvenile kaka were kept captive at the Waiarapa sanctuary along with their parents, from which all four were bred.
Pukaha general manager Helen Tickner said the birds were put down last month after a long consultation process with the Department of Conservation (DoC) and Massey University's Wildbase Hospital.
It was a very difficult decision but the birds weren't thriving and the centre needed to think of the kaka population as a whole, she said.
"They weren't particularly healthy, they weren't thriving and there was no way that they could survive in the wild. With that in mind, they were euthanised ... They were unwell kaka with a hereditary disease that could be passed genetically to others."
There was no cure for the disease and the kaka would have needed ongoing lifelong treatment, she said.
Mrs Tickner said Pukaha did not discuss the decision to euthanise the birds with any environmental groups, "when we're discussing the health and welfare of the animals in our care or that we're breeding, it's not something that we feel we would need to do".
Despite passing on hereditary problems to their four chicks, Mrs Tickner said the parents were well and would be used to parent chicks but would not be bred in the future.
The centre has more than 200 wild kaka.
DoC lists the threat status of kaka as "nationally vulnerable" and estimates the population of the New Zealand native parrot is between 1000 and 5000.